Urban Forest
Research
Center for Urban Forest Research
Summer 2004
• Pacific Southwest Research Station • USDA Forest Service
What Are Your Trees Relative Performance Index?
Annual performance evaluations are
not something we all look forward
to. But at least you know where you
stand after they are over. And your
boss knows how you are performing
relative to the other employees.
A performance evaluation for
trees?
What if you could evaluate the
performance of tree species in your
community? You can. The measure
is called a Relative Performance
Index (RPI). All you need is a basic
inventory. Even a sample inventory
will work. After a simple calculation
you would know how well each tree
species is performing. And most
importantly, how species perform
relative to each other. An RPI could
provide you with a good case for
SPECIES
CONDITION INDEX
Acer saccharinum
Albizia julibrissin
Alnus rhombifolia
Betula pendula
Carpinus betulus
Casurina cunninghamia
Celtis australis
Cercis occidentalis
Celtis sinensis
Fraxinus holotricha Moraine
Fraxinus oxycarpa Raywood
Fraxinus spp.
Fraxinus velutina
Fraxinus velutina Modesto
Ginkgo biloba
Gleditisia triancanthos
Juglans hindsii
Juglans regia
Koelreuteria paniculata
Lagerstroemia indica
Laurus nobilis
Liquidambar styraciflua
Liriodendron tulipifera
Malus floribunda
Magnolia grandiflora
1.1
1.2
0.6
0.4
1.6
1.2
1.4
1.6
1.1
0.8
1.0
0.6
0.1
0.4
1.4
0.7
0.4
1.3
1.4
0.8
1.1
1.2
0.8
1.2
1.5
SPECIES
CONDITION INDEX
Melia azedarach
Morus alba
Pinus canariensis
Pistacia chinensis
Pinus halapensis
Pinus pinea
Platanus acerifolia
Platanus racemosa
Prunus cerasifera
Pyrus calleryana
Pyrus calleryana Aristocrat
Pyrus calleryana Bradford
Quercus agrifolia
Quercus ilex
Quercus lobata
Quercus suber
Quercus virginiana
Rhus lancea
Robinia ambigua
Salix babylonica
Sapium sebiferum
Sequoia sempervirens
Sophora japonica
Zelkova serrata
1.2
1.6
1.5
1.3
1.4
1.5
0.8
1.0
0.5
0.5
1.2
0.9
1.3
1.4
1.0
1.4
1.6
1.1
1.2
1.7
1.0
1.3
0.7
1.1
Table 1. Relative Performance Index for public tree species in Davis, CA
representing over 0.5% of the total population.
“firing” some species and keeping
others. Here’s how it works.
The Davis, CA case
Check out our
website at
http://cufr.ucdavis.edu/
The Davis case started as a master’s
thesis for Scott Maco, urban forester
for our Center. Using a stratified
random sampling technique, he
conducted an inventory of street
trees that included typical
parameters such as species, height,
diameter, age, and condition. His
subsequent analysis of the inventory
data produced a clear picture of the
urban forest. But one particular
aspect of the analysis involved a
seldom used way of looking at street
tree condition data—the RPI.
Relative performance index (RPI)
Relative Performance Index is based
on the proportion of each public tree
species classified as “good”
(continued next page)
2
(continued from previous page)
(condition class) divided by the
proportion of the total population
classified as “good” (Table 1). In
Davis, an index value of ‘1’ indicates
that 60% of that particular species
are in “good” condition, reflecting
the average condition of all species
in the city. A value higher than ‘1’
indicates that there is proportionately more individuals classified as
‘good’. And an index value below ‘1’
indicates that that species has a
below average condition rating when
compared with other street trees.
Condition is the key
Recent pruning and stand age may
be factors, but a tree’s condition is
the overriding indicator of welladapted and appropriate trees. This
was the key, and Scott wanted a
simple way to compare species.
Thus, relative performance index
was born.
Functional life
While a relative performance index
can be used to indicate trees well
suited to your city’s environmental
condition, it is important to remember that some species with low
values may have an age distribution
that represents a senescing population. An example would be many of
Figure 2. Top trees currently planted by numbers and DBH classes in Davis,
CA.
the ash and black walnut in Davis
(Figure 1). Though most of these
trees’ functional lives are past, they
have served the city well throughout
their long lives. And to not replant
these species based on current
condition of these senescing
individuals may be shortsighted.
On the other hand, the fact that
some of the species currently being
heavily planted have values less than
‘1’ suggests one of two things: 1) the
city is either putting faith in species
unlikely to provide stability or costeffective functionality, or 2) proper
maintenance is not being provided,
or it was not provided when the
trees were young. These species—
plum, Bradford pear, crape myrtle,
and plane—are exhibiting relatively
poor condition at young ages,
suggesting that it will be difficult for
them to age gracefully.
Here are some additional factors
to consider when using an RPI:
k Looking at planting trends
Be sure to ask the question: what’s
happening at the small end—the
small diameter classes? What are
you planting? This will directly
(continued next page)
Urban Forest Research
is a publication of the Center
for Urban Forest Research,
Pacific Southwest Research
Station, USDA Forest Service. For more
information, contact the Center at the
Department of Environmental Horticulture,
University of California, 1 Shields Ave,
Suite 1103, Davis, CA 95616-8587. (530)
752-7636
USDA is an equal opportunity provider and
employer, and prohibits discrimination in all
programs and activities.
Figure 1. Age distribution of trees in Davis, CA that are currently producing
the largest average annual benefits on a per tree basis.
Urban Forest Research
Editor: Jim Geiger
Production: Laurie Litman, InfoWright
Summer 2004
3
The shift towards planting small-stature and
untested species has the potential to dramatically
disrupt the current level of benefits
a community enjoys.
affect what becomes larger over
time. For example, in Figure 2 you
can see that recent plantings have
tended toward trees such as London
plane, Chinese pistache, crape
myrtle, Raywood ash and to a lesser
extent Bradford pear, zelkova,
southern magnolia, and purple leaf
plum. Zelkova and perhaps
London plane and Chinese pistache
are the only species with a significant number of individuals present
in functionally large DBH classes
(>12 in.).
k Performance related to benefits
RPI starts with understanding age
distribution for each of your species.
As Scott discovered in Davis,
population stability, and ultimately
stand performance, requires more
than simply planting “other trees”
when a single species is planted
beyond a set threshold (10% of total
population). To our way of thinking,
tree performance is directly related
to a tree’s ability to produce benefits.
Therefore, the more healthy, largestature trees that “grow” into the
larger diameter classes while
maintaining an ideal age distribution
(see Figure 3), the more benefits the
urban forest will provide to the
community.
k Functional size
Functional size is critical too,
because this is when trees are
capable of producing the most
benefits with the least amount of
care. As is evident in Figure 1, large,
long-lived deciduous trees are those
that reach functional size. Substantial tree numbers in large DBH
classes represent proven adaptability. The shift towards planting smallstature and untested species has the
potential to dramatically disrupt the
current level of benefits that a
community enjoys.
The city of Davis stopped plant-
USDA Forest Service
launches Treesearch
Treesearch is an online system for
locating publications by USDA
Forest Service Research and
Development scientists. Publications
in the collection include research by
the Forest Service, as well as papers
written by Forest Service scientists,
but published by other organizations
in their journals, conference proceedings, or books. This website
provides you with an additional way
to find our research documents.
Only peer-reviewed publications will
be housed on Treesearch. All of our
other publications and products are
still available on our website.
Treesearch lets you search
listings by author, keyword,
originating Station, or date. You can
find Treesearch at: http://
216.48.37.142/
ing the majority of species represented in Figure 1—ash, walnut and
locust—due to perceived problems,
whether it was infrastructure or pest
related. It is important, however, to
continue to evaluate how well they,
as well as other species, are aging in
comparison with each other.
The bottom line
Figure 3. Comparative age distribution to the ideal in Davis, CA.
Summer 2004
The RPI may be a completely new
way of looking at your city street
trees, but it provides another
glimpse of what is really going on
underneath the canopy. Try it out.
See if it works for you. And if you
end up “firing” some species and
justifying keeping others you
thought were “over the hill,” our
urban forests will be that much
better off. Let us know how many
species you “fired” and which ones
you “promoted.”
—Jim Geiger
Urban Forest Research
4
How does your tree program “stack up”?
Despite a harsh climate, the city of
Cheyenne, Wyoming has been able
to create an urban forest that has
withstood the test of time. We
recently went to Cheyenne to
conduct a benefit-cost analysis of
their community trees using their
1992 citywide inventory. Check out
the city’s achievements.
k They have 17,010 municipal
trees—48% are publicly managed
and 52% are privately cared for.
k 41% (6300) of all street tree
planting sites are unplanted.
Conversely, the parks are fully
stocked.
k There are 58 different tree
species. Because many species are
newer introductions, and few in
number, overall diversity is low.
k Cottonwood and Siberian elm are
the dominant street tree species,
contributing over 50% of the total
tree leaf area and 60% of the total
canopy cover. Ponderosa/Austrian
pine, blue spruce, and cottonwood
dominate parks.
k The average small or young trees
produce $19/tree in benefits,
maturing medium-sized trees
produce $38/tree, mature large trees
produce $66/tree, and large old trees
produce annual benefits of $80/tree.
k Municipal trees provide
approximately $686 thousand ($40/
tree) in total annual benefits. The
city currently spends about $19/tree
on their care. Citizens are now
receiving a relatively large return on
that investment—$2.09 in benefits
for every $1 spent on tree care.
Future Needs
1. More species diversity. Continued
replacement of senescent
cottonwood and Siberian elm with a
variety of long-lived medium and
Urban Forest Research
Cheyenne, Wyoming
large-stature broadleaf deciduous
tree species will improve diversity.
2. Improved distribution of benefits.
By focusing planting efforts along
streets where stocking levels are
lowest, benefits will be more evenly
distributed.
3. Better management of privatelycared-for trees. Extensive education
on appropriate pruning and irrigation could establish a more consistent management program for street
trees.
Cheyenne is faced with a fragile
resource that needs constant care to
maximize and sustain benefits
through the foreseeable future. In a
city where the climate poses a
constant challenge to tree growth
and health, this is no easy task. The
challenge will be to maximize net
benefits over the long-term,
providing a resource that is both
functional and sustainable.
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Send comments or suggestions to Jim Geiger, Center for Urban Forest
Research, Pacific Southwest Research Station, USDA Forest Service,
c/o Department of Environmental Horticulture, University of California,
1 Shields Avenue, Suite 1103, Davis, CA 95616-8587 or contact
jgeiger@fs.fed.us.
Summer 2004